One of the most spectacular spectacles that Vegas has to offer: and it's free!!! Currently, at least. I'm sure there's some greedy little casino execs in some small room somewhere trying to think of some way to keep the general public out and charge Bellagio prices to view it. Probably all out of spite, because they're 40 year old virgins, and even the Vegas escorts turn them down.
Definitely catch at least one of the shows, because it is amazing, and you never know when they'll pull the plug on it. Vegas doesn't give a GFE anymore. They want your money, and that's that. Also consider dining at one of the waterfront restaurants inside Bellagio during the evening, and you'll get to see the various fountain shows. It's a must do.